arc4random.h: remove some cruft.

This hasn't been used since the code was rewritten years ago.

Test: builds
Change-Id: I2c4bccb3fffb15115083afbb178519bd133c64de
diff --git a/libc/upstream-openbsd/android/include/arc4random.h b/libc/upstream-openbsd/android/include/arc4random.h
index 5f0b15e..96f94be 100644
--- a/libc/upstream-openbsd/android/include/arc4random.h
+++ b/libc/upstream-openbsd/android/include/arc4random.h
@@ -36,14 +36,6 @@
 //#define _ARC4_LOCK()   pthread_mutex_lock(&arc4random_mtx)
 //#define _ARC4_UNLOCK() pthread_mutex_unlock(&arc4random_mtx)
 
-#ifdef __GLIBC__
-extern void *__dso_handle;
-extern int __register_atfork(void (*)(void), void(*)(void), void (*)(void), void *);
-#define _ARC4_ATFORK(f) __register_atfork(NULL, NULL, (f), __dso_handle)
-#else
-#define _ARC4_ATFORK(f) pthread_atfork(NULL, NULL, (f))
-#endif
-
 static inline void _getentropy_fail(void) {
     async_safe_fatal("getentropy failed: %s", strerror(errno));
 }